Electron beam inspection

ABSTRACT

Differences between the secondary electron emission characteristics of two phases of the same material in response to bombardment by an electron beam are sensed to enable impurities, comprising one of the material&#39;&#39;s phases, in a sample, comprising the material&#39;&#39;s phase, to be detected. The electron beam is a low energy beam. Relative motion between the sample and the beam may be effected by sweeping the beam, by moving the sample, or by a combination of the two motions.

United States Patent Dugan 1 Nov. 21, 1972 [54] ELECTRON BEAM INSPECTION3,473,023 10/1969 Bloch ..250/49.5 PE [72] Inventor: gs z ifag yggfi m gPrimary Examiner-James W. Lawrence a Assistant Examiner-C. E. Church[73] Assignee: Western Electric Company, lncor- Attorney-W. M. Kain, R.P. Miller and R. C. Winter porated, New York, NY. 22 Filed: Nov. 23,1970 [57] ABSTRACT Differences between the secondary electron emission[211 Appl' 91654 characteristics of two phases of the same material inresponse to bombardment by an electron beam are [52] user. ..2s0/49.sPE,250/49.5 A sensed to enable impurities, comprising one of the [51]Int. Cl. ..G0ln 23/04 materials P a sample ""P materi- [58] Field ofSearch ..250/49.5 PE 49.5 A Phase be detected- The elem beam is a energybeam. Relative motion between the sample and [56] Reerences Cited thebeam may be effected by sweeping the beam, by moving the sample, or by acombination of the two UNITED STATES PATENTS motions.

3,479,506 11/1969 Dorfler ..250/49.5 PE 4 Claims, 10 Drawing Figures I05OO 144 U llk 105 J K I06 i V [lo l,

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nr 0 u w .J o O l l l l 0 IO 7 --4O GRID BIAS (VOLTS) m m z o m 5 5 n:0.5 7 :7 g: 7 i1 5 w Lu SECONDARY ELECTRON ENERGY(eV) SECONDARY 4*PRIMARY 1 ELECTRON BEAM msrscrron BACKGROUNDOF THE INVENTlON 1. Field ofthe invention The present invention relates to the detection ofimpurities in a sampleand, more particularly, to a method ofelectrically detecting, in a sample, the presence of impurities, thesample being a first phase of a material while the impurities are asecond phase of the same material.

In a more specific sense, the present invention contemplates inspectinga sample of a first phase of material to detect the presence therein ofa second phase of the same material by using techniques derived fromscanning electron microscopy (SEM).

2. Discussion of the Prior Art The use of SEM techniques to study bothsurface topography and the boundary between a first material (e.g.,silicon) and a second material (e.g., gold), deposited on the first, iswell known. In typical SEM techniques a sharply focused beam of highenergy elec' trons, usually in the range of 3 keV-40keV, is movedrelatively with respect to a sample, e.g., by sweeping the beamthereover in a raster. impingement of the beam on the sample effects oneor more of several types of beam-induced signals. These signals may beutilized to form an image by means of a sweep on a cathode ray tube(synchronized with the beam sweep), modulated by the beam-inducedsignals.

The beam-induced signals may be generated by emitted secondaryelectrons, emitted back-scattered electrons, emitted auger electrons,emitted photons, or may be proportional to various types of currentsinduced in the sample, all of these being due to impingement of the beamon the sample. It has been generally preferred to form the image on acathode ray tube with signals generated by emitted secondary electrons,back-scattered electrons, or induced currents.

Notwithstanding the rather highly developed state of the SEM art, assummarized above, there have been no known techniques for accurately andreproducibly dc tecting or observing the presence of a second phase of amaterial in a sample of a first phase of the same material. It is notknown why such a deficiency in the SEM art exists. Nevertheless, oneobject of the present invention is to enable such detection to beeffected.

The detection of sites of the second material phase, herein termedimpurities, in the first material phase, herein termed the sample, isespecially important, when the phases are different phases of tantalum.This is true when the sample is B tantalum (tetragonal) and theimpurities are body centered cubic (bcc) tantalum, hereinafter called atantalum. There is also importance in the detection of B tantalumimpurities in a tantalum samples.

Today, sputtered B tantalum is considered a preferred material in theproduction of thin-film capacitors. Its electrical properties,especially its dielectric properties when anodized, have led to thispreferred status. See US. Pat. Nos. 3,391,373; 3,382,053; and 3,275,915.Generally, such thin-film capacitors comprise a sputtered film orthin-film of B tantalum on a substrate, a portion of the film beinganodized to form a B tantalum pentoxide dielectric layer. This layer isultimately covered with a metallic film (often gold) which serves as acapacitor countere- 2 lectrode. Thus, the completed capacitor comprises,in order, a substrate, a B tantalum film, a B tantalum pentoxide layerand a counterelectrode.

Sputtered B tantalum films have been found to contain, at times, atantalum inpurities, which impurities undesirably affect the otherwisedesirable electrical characteristics of the B tantalum. Specifically,when B tantalum films containing a tantalum impurities are anodized toproduce the a tantalum pentoxide capacitor dielectric layer, theanodized a tantalum sites result in a tantalum pentoxide sites whichhave a different (usually lower) dielectric constant than the B tantalumpentoxide layer. Further, the a tantalum pentoxide sites often (aboutpercent of the time), for unexplained reasons, result in leakycapacitors and/or capacitors which break down at their intended voltage.

The presence of such a tantalum impurities is thought to be caused byimproperly adjusted parameters of the sputtering procedures by which Btantalum films are usually produced. The exact nature of, and theinteraction between, these parameters is not presently understood,however. Accordingly, at times these parameters can be adjusted toeliminate such a tantalum impurities; however, at other times suchelimination proves difficult or impossible. Nevertheless, the detectionof the a tantalum impurities prior to anodization of the B tantalum filmat least prevents additional processing (e.g., anodization,counterelectrode deposition, etc.) of a B tantalum film which cannot beused to produce an acceptable capacitor.

Thus, the accurate, simple and expedient detection of a tantalumimpurity sites in B tantalum thin films is a sought-after end, and is anobject of the present invention.

B tantalumfilms for capacitors are usually produced by a so-calledin-line sputtering machine in which a conveyor-like track continuouslytransports substrates through one or more sputtering environments andlow pressure chambers which serveas air-locks. See US. Pat. No.3,340,176 (assigned to the assignee of the present invention) for adisclosure of one type of such an in-line sputtering machine. Continuousinspection of the films as they leave the machine is a desirable end andan object of the present invention. Another object of the presentinvention is to utilize improved SEM techniques to realize this end.

Specifically, as noted before, SEM techniques utilize relative motionbetween the sample and the electron beam. Either the sample or the beamor both may be moved, although it is more typical to sweep the beam in araster. Because the sputtered B tantalum films leave the in-line machinecontinuously on the track (in an arbitrarily chosen Y-direction ofmotion) the electron beam can be simply swept (in a X-direction)thereover. The signals produced by such scanning, with propercoordination of the scan rate and the track speed, can be used to adjustthe sputtering parameters, or at least to pinpoint B tantalum films witha tantalum impurities therein. Such is also an object of the presentinvention.

it should be noted that considerations similar to the above, also obtainwhen a tantalum films are being produced. At times B tantalum impuritiesreside therein, and detection of such impurities is yet another objectof this invention.

Moreover, the detection of impurities (i.e., one phase of a givenmaterial, not necessarily tantalum) in a sample (i.e., another phase ofthe same given material, again, not necessarily tantalum) is a generalobject of this invention.

SUMMARY OF THE INVENTION With the above and other objects in view, thepresent invention contemplates a new and improved method of electricallydetecting impurities in a sample of a predetermined material. Theimpurities comprise one phase of the material and the sample comprisesanother phase of the same material. For example, the sample may be inthe form of a sputtered thin film of 6 tantalum and the impurities maybe a tantalum sites. The composition of the sample and the impuritiesmay be reversed, (i.e., the sample may be a tantalum and the impuritiesmay be B tantalum.

A low energy electron beam is generated, the electrons therein havingenergies within the range of 0.1-3.0 keV. Preferably, however, theenergies of the majority of the beam electrons are at approximately 0.4keV. The electron beam is swept across the sample, either in a rasterscan or as a result of the combined motion of the beam and of thesample. Sweeping of the beam effects the emission from the sample of,inter alia, secondary electrons and back-scattered electrons. Theemitted secondary electrons are captured and a current is derivedproportional to the number of secondary electrons so captured.

It has been found that different phases of the same material, whethertantalum or not, emit substantially different numbers of secondaryelectrons (but about the same number of back-scattered electrons) uponthe low energy electron beam impinging thereon. Accordingly, differencesin the number of secondary electrons captured yields a direct indicationof the presence of the second phase impurities in the sample of thefirst material phase.

Accordingly, in synchronism with the relative motion between the beamand the sample, variations in the derived current are measured. Suchmeasurement indicates changes in the number of secondary electronscaptured which changes, in turn, are indicative of the number ofsecondary electrons emitted.

It has also been found that different phases of the same material emit,upon low energy electron bombardment, substantially the same number ofback-scattered electrons. Accordingly, one of two alternative techniquesmay be used in carrying out the above method. First, the capturing stepmay be effected so that almost all of the back-scattered electrons areexcluded. Such exclusion is based on the fact that the geometry of theemitted secondary and back-scattered electron configurations renders itpossible to capture either one or the other or both types of emittedelectrons. Second, where more convenient, the capturing step may effectthe capture of both the secondary and the back-scattered electrons. Inthis instance, because as noted above, different phases of the samematerial emit substantially the same number of back-scattered electrons,the derived current may have subtracted therefrom a constant currentproportional to the number of back-scattered electrons. In either case,the ultimate derived current is proportional to the number of secondaryelectrons and pennits the convenient detection of the presence ofimpurities in the sample.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thepresent invention will appear upon a consideration of the followingdetailed description with the accompanying drawings wherein:

FIG. la is a schematic diagram of low energy scanning electron beaminspection apparatus, used in carrying out the method of the presentinvention, to detect impurities in samples;

FIG. lb is an alternative embodiment of FIG. la wherein the inspectionapparatus thereof is used in conjunction with an in-line sputteringmachine;

FIG. 2 is a schematic diagram of apparatus used to measure the secondaryand back-scattered electron emission properties of a typical materialcomprising a sample which is to be inspected and of a typical materialcomprising an impurity in the sample which is to be detected by theapparatus of FIGS. la or lb;

FIG. 3 is a graph showing the back-scattered electron emissioncoefficient 1 versus the incident electron beam energy obtained with theapparatus of FIG. 2 for the material comprising the sample to beinspected by the apparatus of FIGS. la or lb and for the impurities inthe sample;'

FIG. 4 is a graph depicting the secondary electron emission coefficient8 versus the electron beam energy obtained with the apparatus of FIG. 2for the material comprising the sample to be inspected by the apparatusof FIGS. la or lb and for the impurities therein;

FIG. 5 is a graph of the ration of 8 to 1 for both the materialcomprising the sample and the impurities therein versus the electronbeam energy obtained by combining the graphs of FIGS. 3 and 4 and usingaverage values for 8 and 1 therefrom;

FIG. 6 is a graph depicting, in the apparatus of FIG. 2, the currentthrough an emitted electron collector as a function of the bias on aretarding grid when a 1.5 keV electron beam impinges normally on thematerials comprising the sample and the impurities therein;

FIG. 7 is a graph depicting the ratio 6/eV versus the electron energy ofsecondary electrons emitted from both materials comprising the sampleand the impurities therein upon bombardment thereof by a 1.5 keVelectron beam nonnal thereto; this graph was obtained by differentiationof the graph of FIG. 6;

FIG. 8 is a graph depicting the variation of 8 for the material of thesample and of the impurities therein as a function of the angle ofincidence of a 1.5 keV electron beam with respect thereto wherein 0 isnormal incidence;

FIG. 9 is an enlarged view of a thin film of B tantalum having atantalum impurities therein, these two phases of the same material beingtypical of the materials of the sample and impurities, respectively, tobe inspected by the apparatus of FIGS. la or lb.

DETAILED DESCRIPTION Before discussing the method of the presentinvention and the apparatus for effecting that method, it is necessaryto discuss some preliminary considerations. These preliminaryconsiderations concern certain characteristics of a sample 14 to beinspected and of impurity sites 15 therein to be detected.

The sample 14 to be inspected by the present method may be in any form.Referring to FIG. 9, there is an exemplary sample 14 comprising a thinfilm 16 deposited, for example, by sputtering on a glass or otherappropriate substrate 17,. The thin film 16 contains the impurity sites15, the size of which is greatly exaggerated in FIG. 9. Of particularinterest in the present invention are impurities 15 which are both adifferent phase of the material of the film l6 and undetectable byordinary visual techniques, as well as impurities which are visuallydetectable. Exemplary materials are [3 tantalumfor the film 16(ultimately used as a capacitor electrode) and a tantalum for theimpurities 15 (which may prevent the film 16 from being properlyanodized to form the capacitor). it is to be noted that different phasesof the same material, whether tantalum or some other material, haveapproximately the same atomic number Z and that the impurities 15 ofinterest here are those which may be undetectable by ordinary visualmeans.

When an electron (hereinafter called a primary electron) of energy E, isincident upon a target, a number of different types of electrons (andphotons, xrays, etc.) are emitted therefrom. This is true whether thetarget is the material of the film 16 of the sample 14 or is thematerial of the impurities 17 in the film 16. Two types of emittedelectrons of interest here are back-scattered electrons and secondaryelectrons, for reasons to be more fully discussed below. For purposes ofthe present invention, if the film 16 is B tantalum and the impurities15 are or tantalum, emitted electrons having energies greater than 40eVare considered back-scattered electrons while those electrons havingenergies less than 40eV are considered secondary electrons. This limitis somewhat arbitrary and will vary according to the material of thefilm 16 (one phase) and of the impurities 15 the other phase). Thechoice of this limit is discussed below with reference to H6. 6.Nonetheless, in accordance with standard definitions, secondaryelectrons are considered to be of lower energies than back-scatteredelectrons.

Back-scattered electrons originate near or at a surface of the targetupon which the primary electron of energy E, is incident. Manyback-scattered electrons are incident electrons which undergo nearlyelastic reflection from the first few atomic layers of the targetsurface. It has been found that the number of backscattered electronsemitted from the surface for a given incident primary electron energyis, for the most part, some function of the atomic number Z of thematerial of thesurface. Moreover, different phases of the same materialhave substantially the same atomic number Z. Accordingly, if a surfaceupon which an electron is incident contains impurities of a differentphase of the same material which constitutes the surface, substantiallythe same number of back-scattered electrons will be emitted from boththe surface and the impurities therein, as shown in FIG. 3, discussedbelow.

Secondary electrons are emitted from a surface when an incident electronof energy E, penetrates the surface and is scattered within the materialconstituting the surface. Such scattering transfers energy from theincident electron to other electrons within the material. Many of theseelectrons to which energy is transferred migrate to the surface and areemitted from the surface if they are energetic enough to penetrate thepotential barrier of the surface. It has been found that the number ofsecondary electrons emitted is dependent upon, among other things,parameters such as the average depth beneath the surface at which thesecondary electrons are generated, the condition of the surface and thework function of the material of the surface. Assuming these factors tobe generally equal, it has been found that different phases of the samematerial emit, upon being bombarded by low energy electron beam,substantially different numbers of secondary electrons, as more fullydiscussed below with reference to FIG. 4.

For purposes of this invention, a secondary electron emissioncoefficient 8 isdefined as the ratio of the number of secondaryelectrons emitted to the number of primary electrons incident upon asurface. Similarly, a back-scattered electron coefficient 'r is definedas the ratio of the number of back-scattered electrons emitted from thesurface to the number of primary electrons incident thereupon. Theincident electrons are referred to-as primary electrons. Referring nowto FIG. 2, there is shown apparatus for performing certain preliminarysteps of the method of the instant invention. These preliminary stepsinvolve an initial characterization of the materials comprising thesample 14, specifically, the material of the film l6 and'of theimpurities 15 therein which are a different phase of the same material.For convenience {3 tantalum and a tantalum are chosen for suchmaterials, but similar characterizations may be easily effected fordifferent phases of materials other than tantalum.

Within a chamber21l evacuable to a low pressure by standard facilities,not shown, there is contained a target holder 22, a standard electrongun 24, and an emitted electron collector system 26. The electron gun 24is so positioned that a beam of low energy electrons 28 may be directedat the target holder 22 at varying energies and angles of incidence. Theelectron gun 24 is shown generally and may include standard facilitiessuch as a cathode, an anode, electronic optics, deflection plates, etc.,not shown.

The target holder 22 may comprise a conductive member 30 rotatable on anaxis 32 thereof by means, not shown. The member 30 may take anyconvenient shape, a generally square cross-section being shown in FIG.2. Mounted to one face A of the member 30 is a faraday cup 36 which is avery efficient electron collector, as is well known. Mounted to anotherface B of the member 31) is a target 41). The target 40 includes,depending upon the characterizations being effected, either a deposit 42of the essentially pure material of the film 16 of the sample 14 to beinspected or of the material of the impurities 15 to be detected byinspection of the sample 14, the impurities 15 being a different phaseof the same material as the sample film 16.

The purity of the target 4% is ascertained by standard methods.

The deposit 42 may reside on a substrate 17, which may be the same asthe substrate 17 of FIG. 9. Moreover, the deposit 42 may convenientlybe, but not necessarily, a thin film. Specifically, if the sample 14includes a sputtered thin film (of B or a tantalum) so too should be thedeposit 42 of the target 40; conversely, if the sample 14 includes anelectrodeposited thick film, the deposit 42 of the target 40 should besuch a thick film; etc. Moreover, the deposit 42 is electricallyconnected to the member 30 via a convenient conductor such as at 44.

Selective rotation of the member 30 about the axis 32 positions one ofthe two faces A or B thereof to have incident thereon the electron beam28 from the electron gun 24. The member 30 is grounded, as shown, inseries with a first electrometer 46 or other current measuring device.

The electron collector system 26 includes a hemispherical, conductivedome 48 with its open, concave side facing the target holder 22. Mountedthrough the dome on a radius thereof is a standard drift tube 50 throughwhich the electron beam 28 passes in traveling to the target holder 22.The drift tube 50 is well known in the art, and merely prevents theelectron beam 28 from being adversely affected by varying potentials onthe various elements of the electron collector system 26.

Mounted within the concave side of the dome 48 and insulatively spacedtherefrom is a suppressor grid 52.

The dome 48 is grounded in series with a first variable voltage source54 and a second electrometer 56 or other current sensor. The suppressorgrid 52 is connected to a movable arm 58 of a double pole switch 60.When the arm 58 engages a first contact 61 of the switch 60, thesuppressor grid 52 is grounded in parallel with the dome 48. When thearm 58 engages a second contact 62 of the switch 60 the grid 52 isgrounded in series with a second variable voltage source 64 and a thirdelectrometer 66 or other current sensor.

The variable voltage source 54 has its positive side connected to thedome 48 and the contact 61, while the variable voltage source 64 has itsnegative side connected to the contact 62.

In general, if a grounded object, such as the deposit 42 of the target40, is bombarded by an electron beam 28 of energy E, a relationshipexists between the beam current i,,, the current flowing between theobject and ground i the current due to back-scattered electrons i andthe current due to secondary electrons i That relationship is:

It can be seen that measurement of any three of these currents permits adirect determination of n and 8. Specifically, this is true because thenumber of primary electrons is proportional to i,,, n is proportional toi,,, and 8 is proportional to i In operating the apparatus of FIG. 2,the member 30 of the target holder 22 is rotated on the axis 32 to firstposition the faraday cup 36 to receive the electron beam 28. The arm 58is moved to engage the contact 61 and the dome 48 and the grid 52 arerendered positive (attractive of electrons), for example, by applyingthereto a potential of about 120 volts from the source 54. Thus, shouldany secondary or back-scattered electrons be emitted by, or escape, thefaraday cup 36 they are attracted to and captured by the dome and grid48 and 52, as will be indicated by current flow through the electrometer56. The electron gun 24 is turned on. The current i, to ground throughthe electrometer 46 for the faraday cup 36 (i, i is now measured as isthe current (i +1, through the electrometer 56.

It is found, as should be expected, that very few electrons are emittedby or escape the faraday cup 36. Notwithstanding the high positivepotential on the dome 48 and on the grid 52, the electrometer 56indicates substantially zero current flow to ground. Accordingly, whenthe faraday cup 36 is bombarded by the electron beam 28,

i.,+i,=0;thus,' (2a) This value for i, (=i is used subsequently indetermining the value of 8 and 1 Specifically,

n=i.,/i,,=i,/i (2d) Next, the member 30 is rotated to position thedeposit 42 of the target 40 so that the electron beam 28 impingesthereon. As noted previously, the deposit 42 may include either asubstantially pure thin film of the material of the film 16 (B tantalum)or of the material of the impurities 15 (a tantalum). Bombardment of thedeposit 42 which comprises the film l6 material by the electron beam 28results in the flow of a current through the electrometer 56, the arm 58remaining in engagement with the contact 61 of the switch 60 to applythe positive potential to both the dome 48 and the grid 52. Thiscurrent, denoted i, and due to electrons captured by the positivelybiased dome 48 and grid 52, contains components due to both secondaryand back-scattered electrons. Specifically, i, is defined as ic=ii+ii.(a) the positive bias (here, volts) being sufficiently high to effectcapture of substantially all the emitted secondary and back-scatteredelectrons. This bias due to the source 54, may, of course, be varieddepending on the material of the deposit 42, the only requirement beingthat substantially all of the emitted secondary and back-scatteredelectrons are captured.

Next, the arm 58 is moved to engage the contact 62. Initially the source64 is adjusted to produce zero (0) volts output. Then the source 64 isadjusted to render the grid 52 increasingly more negative while the dome48 remains biased positively. Depending upon the material of the deposit42, the voltage source 64 is now adjusted until all of the lower energysecondary electrons are repelled from the electron collector system 26by the negative bias on the grid 52. The higher energy back-scatteredelectrons get through the negatively biased grid 52 to the positivelybiased dome 48.

At the grid bias where all or substantially all secondary electrons arerepelled, both electrometers 56 and 66 indicate current flow denoted i,and i respectively, due to the more energetic back-scattered electronsreaching the dome 48 and the grid 52, respectively.

After the above current measurements have been made, it is a relativelysimple matter to obtain the values of 6 and 1 as follows:

Because the current i is due solely to back-scattered electrons, i isdefined as Thus, from equation (3),

9 i i, i and from equations (4) and (5 i, ='i, (i +i,"). 6

Dividing equations (4) and (6) by 1, (previously defined as i, i,,) inaccordance with equations (2c) an (2d),

Note that it here is the actual i, measured depending on the characterof the target 40, while the i, (i, i used previously is that presentwhen the faraday cup 36 receives the beam 28 (in the latter i is usedonly to ob tain a measure of i,,).

Accordingly, from equation (ll) measurement of i, (by a meter, a CRT,etc.) gives an indication of i,, albeit a mirror measurement.

Referring now to FIG. 3, obtained by the abovedescribed use of theapparatus of FIG. 2, there is shown a graph representing the varyingback-scattered electron emission coefficient 1 versus the electronenergy li of the electron beam 28 for both the material of the film 16and the material of the impurities 15. This graph contains informationrelating to B tantalum, which is an example of the material of the film16, and a tantalum which is an example of the material of impurities 15.Because the impurities are merely a different phase of the material ofthe sample 15, as may be seen from FIG. 3, the value of 'n (as well asi, i, i,") for varying primary electron energies E, from about 0.2-2.0keV for the two is substantially the same. Accordingly, as previouslydiscussed, the use of backscattered electrons to detect the presence ofthe a tantalum impurities 17 in the B tantalum sample 15 is exceedinglydifficult. Again, this difficulty is due to the fact that the atomicnumbers Z of both phases of tantalum are substantially the same. Formaterials other than tantalum, this similarity of 'n is experienced overan E, range ofO. l-3.0 keV.

Referring now to FIG. 4, also obtained via the abovedescribed use of theapparatus of FIG. 2, there is shown a graph similar toFIG. 3 except thatthe value of 1; for both tantalum phases is plotted against the energyE, of the electrons in the beam 28. As can be seen from the figure, forelectron energies between 0.2-2.0 keV, the value of 8 for a and Btantalum is significantly different. The difference in the respectivevalues of 6 is a maximum at incident electron energies at about 0.4keVand is a minimum at energies greater than 2.0keV. For non-tantalummaterials the E, range of 0.1-3.0 permits easy difi'erentiation of 8 forthe two phases. The maxima and minima 8 differences are at a differentE,,, which can be easily calculated as discussed below.

In fact, referring to FIG. 5 (a combination of FIGS. 3 and 4) which is aplot of the ratio of 8 to 1 (both averaged for both materials) versusthe energy of the electrons in the beam 28, it may be seen that, atabout 0.4keV, 8 is nine times greater than n. Thus, it may be said thatwith the same amount of noise present in measuring either 8 or n, thesignal-to-noise ratio of the 8 measurement is'nine times greater thanthe signal-tonoise ratio of the '1 measurement, at beam electronenergies of 0.4keV. .At other beam electron energies, thesignal-to-noise ratio, while less than 9, is about 3 or greater.

Referring now to FIG. 6, there is shown a graph of (a) the current (i, ithrough the electrometers 56 and 66 when the arm 53 engages the contact62 versus (b) the bias on the grid 52 due to adjustment of the variablevoltage source 6 3 for both a and B tantalum. There is, as can be seen,a measurable difference between the values of the collector currentcurves for the two materials up to about 40 volts bias on the grid 52..The assumption is made here that this difference is due to secondaryelectron emission differences between the two materials, theirback-scattered electron differences, as shown by FIG. 3, beingnegligible and both varying substantially linearly. The fact that at anegative 40 volt bias the curves merge leads to the conclusion that, atleast for purposes of a and h tantalum, secondary electrons are thoseelectrons having energies less than about 40eV. Such a definition ofsecondary electrons is accordingly used herein with reference to a andI8 tantalum. Specifically:

Secondary electrons energies less than 40eV Back-scattered electronsenergies greater than In fact, referring to FIG. 7, tests show that theenergy distribution of the secondary electrons emitted from a and 3tantalum is one wherein most of these electrons have energies within theapproximate range O-ISeV, with the majority of these being at about 4eV.

From materials other than a and B tantalum, variations in the graphs ofFIGS. 3-7 will exist but the result is the same. Where the materials aredifferent phases of the same material, a graph similar to MG. 3, usingthe apparatus of FIG. 2, shows that 11 for the two materials is aboutthe sameand varies linearly for'low, varying E,s. Moreover, 8 for thesame 18, will be measurably different for the two phases, similar toFIG. 4. Also, a graph similar to FIG. 5 will show significantsignal-tonoise ratio improvements of 8 over 1 Moreover, use of theapparatus of FIG. 2 produces graphs similar to FIGS. 6 and 7, indicatingthat at low primary electron energies E,, an easy differentiationbetween secondary and backscattered electrons can be made.

It should be noted that FIGS. 3-7 result from the bombardment of a and)3 tantalum deposits 42 in the target 40 with an electron beam 28 of1.5keV. It has been found that substantially the same curves, withtypical shifts of only fl percent are generated when the electron beam28 energy E, is within the preferred range of 0.2-2.0 keV. Similarminimal shifts are observed for materials other than tantalum.

FIG. 8 shows the variation in 6 for a and B tantalum with a 1.5keVelectron beam 28 as the angle of incidence of the beam 28 with respectto the target 40 is varied. Note that represents a beam 28 perpendicularto the target 40. The curves for the two materials indicate that theangular dependence of 8 for both tantalum phases is similar. Both 8sincrease about 40 percent as the incident angle increases from 0-40.Tests indicate that the shape of these curves are substantiallyindependent of the energy of the beam 28 over the range 0.22.0 keV.Similar results are obtained with non-tantalum materials.

Thus, it is worthy of note that:

' a. at low (0.22.0 keV) E,, 17 for two different phases of the samematerial is practically indistinguishable (FIG. 3); the same issubstantially true at E, greater than 2.0keV;

b. at low (0.2-2.0keV) E,, 8 for the two phases is different and thedifierence is easily detectable (FIG. 4); at E, greater than 2.0, 8differences are practically indistinguishable;

c. at low (0.2-20 keV) E,, 8/1; is 3 to 9, indicating 8 is a betterindicator of detection with a given amount of noise present (FIG. 5);

d. a sharp line of practical demarcation exists at low (02-20 keV) andE, may be easily defined between secondary and back-scattered electrons(FIGS. 6 and 7); and

e. statements (a) (d), above, are independent of the angle of incidenceof an electron beam of low (0.22.0 keV) E,.

Referring now to FIG. 1, there is shown one embodiment 100 of the lowenergy scanning electron beam inspection system of the presentinvention. This system 100 resembles a scanning electron microscope inthat it uses a scanning electron beam as a probe.

An electron beam generator 102 comprises the usual electron source 103,grid 104, anode 105 and first and second pairs of deflection plates 106and 107, respectively. The deflection plates 106 control the sweep ofthe beam 110 in X direction; the plates 107 control the Y sweep. Anelectron beam 110 generated by this arrangement 102 is directed at andimpinges on the sample 14.

Knowing that the secondary electron emission coefficient 8 of thematerial of the film 16 and of the impurities therein (a different phaseof the same material) are quite different at low primary electronenergies E, permits detection of these impurities by a detectorsubsystem.

The detector sub-system includes a secondary electron collector 122,which comprises a conventional scintillator 123, a light pipe 124 andphotomultiplier 125 which produces, at an output 126 thereof, a currentproportional to the number of secondary electrons emitted by thebombarded sample 15. The scintillator 123 may be positioned so that itreceives only secondary electrons and very few back-scattered electrons,as is known.

Another arrangement, not shown, would be to position the scintillator123 to receive both secondary and back-scattered electrons and to makean adjustment in the current of the output 126 to eliminate thecomponent (i.e., i," i,") of that current due solely to theback-scattered electrons. As noted previously, use of the apparatus ofFIG. 2 permits calculation of i," i," for any E,. Accordingly, theconstant i, i," i," may be continuously subtracted from the currentoutput 126 of the photomultiplier. Because i, is the same for bothphases of the material in the sample 14 at low E,, only i, differencesare impressed on the dual amplifier 150.

X-ramp and Y-ramp generators 128 and 130, respectively, have theiroutputs 132 and 134, respectively, connected to an X-magnificationcontrol 136 and a Y- magnification control 138. The X-magnificationcontrol 136 is connected to the deflection plates 106 to control the Xmotion component of the raster scan of the electron beam on the sample14. The Y-magnification control 138 is connected to the deflectionplates 107 to permit control of the Y motion component of the rasterscan of the electron beam 110.

Outputs 132' and 134', respectively, of the X- and Y- ramp generators128 and 130, respectively, are connected to an amplifier section 140.The amplifier section 140 includes an amplifier 141 coupled to theoutput of the X-rarnp generator 128. The output 142 of the amplifier 141is connected to deflection plates 144 in a standard cathode ray tube 146for controlling the X scan thereof. Thus, the signal applied to theplates 144 is the same as that applied to the plates 106 (divided, ofcourse, by any magnification factor supplied by the control 136).

The Y scan of the cathode ray tube 146 is controlled by deflectionplates 148. The deflection plates 148 are controlled by the output 149of a dual amplifier 1.50 in the amplifier section 140.

The dual amplifier 150 receives the output (on 134) of the Y-rampgenerator 1.30 and the output 126 of the photo-multiplier 125. Thetime-varying output of the Y-ramp generator 130 is amplitude-modulatedby the output of the photo-multiplier 126 in the dual amplifier 150.Accordingly, the signal applied to the deflection plates 148 is anamplitude-modulated signal which at"- fects the Y component of theraster scan of the cathode ray tube 146 proportionally to the number ofsecondary electrons emitted by the sample M. Of course, the Y-rampcomponent applied to both sets of plates 107, M8 is the same, exceptfor, in addition to modulation, any magnification supplied by thecontrol 130.

Due to the differences in the secondary electron emissioncharacteristics of B and a tantalum, as previously described, as theelectron beam 110 scans across the film 16 and the impurities 15 in thatfilm 16 areas of a and B tantalum produce visibly different outputs onthe cathode ray tube 146.

Suitable results are also obtained if an alternative embodiment 200 isused in conjunction with an inline sputtering machine 201. Such use ismost convenient, because the output end 202 of the in-line machine 201is maintained in an evacuated condition. Accordingly, the electron beamgenerator 102 may be placed physically in one of the evacuated chambers203 near the output end 202 of the in-line machine 201.

Because the samples 14 move continuously through the in-line machine, asingle set of deflection plates, for example, the X plates 106, may beused. Specifically, the X plates may be used simply to scan the beam 110back and forth (with blanking, if desired) across the samples 14 as theymove, due to the movement of a conveyor-like track 204'through thechamber 203, Y scanning being provided by the motion of the samples 14.An appropriate sensor 204 may be used to sense the speed of the track204 (and of the samples 14) to provide a Y signal to the dual amplifier150. This Y signal is modulated by the output 126 of the photomultiplier125 to effect deflection of the beam in the CRT 146 via deflectionplates 148. Such sensor 204, accordingly, may replace the elements 107,138, 13 0, and 130 and 134' in FIG. 1. Of course, Y scanning may beprovided as in FIG. 1.

It should be obvious to one skilled in the art that the CRT 146 is notan essential element of the present invention. Having a signal availableat the output 126 of the photomultiplier 125 proportional to 8 permitsthe use of other expedients such as meters, etc., to detect theimpurities 15. In fact, the output 126 may be conveniently connected toa computer facility, programmed to make judgements concerning theacceptability of the samples 14 based on the number of impurity sitesl-present in the film 16.

In any event, the output 126 may be used either to:

a. adjust the sputtering parameters of the inline machine 201 toeliminate the unwanted impurities 15, or, where such elimination isimpossible because of unknown factors,

b. detect and prevent the further processing of films 16 having suchimpurities.

It is to be understood that the above-described embodiments are simplyillustrative of the principles of the invention. Various othermodifications and changes may be devised by those skilled in the artwhich will embody the principles of the invention and fall within thespirit and scope thereof.

Whatis claimed is:

1. A method of detecting, in a sample of a first phase of a material,the presence of a second, different phase of the material, said firstand second phases having the same atomic number Z, which comprises thesteps of:

impinging said sample with a low energy electron beam, the electrons insaid beam having energies falling within the approximate range 0.2-2.0keV for effecting the emission from said sample of secondary electrons,substantially all of which have energies within the range 0-15 eV, andbackscattered electrons in approximately equal numbers from both phasesof said material;

capturing said secondary electrons to the substantial exclusion of saidback-scattered electrons to thereby derive a current proportional to thenumber of said emitted secondary electrons; and relatively moving saidbeam and the sample and simultaneously detecting, in synchronizationwith said relative motion, variations in said derived current f0indicating changes in the number of said emitte secondary electrons todetect the presence of the second material phase in the sample.

2. The method of claim 1 wherein said material is tantalum and saidfirst and second phases are respectively the a and )3 phases thereof.

3. The method of claim 1 wherein said material is tantalum and saidfirst and second phases are respectively the B and 0: phases thereof.

4. The method of claim 1 wherein the energy of the majority of theelectrons in said beam is approximately 0.4 keV.

L-566-PT Patent No. Dated November 21,

Inventor(s) Allan Edward Dugan It is Certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

r In the specification, Column 3, line 18, (i.e. should 1 read --i.e.Column 9, line 1, the equation should read i i i -5 line 29, "i is aconstant" should read "1, is a constant--; line 36, i i K K K should '-jt K3|'"''o Signed and sealed this 3rd day of April 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. A method of detecting, in a sample of a first phase of a material,the presence of a second, different phase of the material, said firstand second phases having the same atomic number Z, which comprises thesteps of: impinging said sample with a low energy electron beam, theelectrons in said beam having energies falling within the approximaterange 0.2-2.0 keV for effecting the emission from said sample ofsecondary electrons, substantially all of which have energies within therange 0-15 eV, and back-scattered electrons in approximately equalnumbers from both phases of said material; capturing said secondaryelectrons to the substantial exclusion of said back-scattered electronsto thereby derive a current proportional to the number of said emittedsecondary electrons; and relatively moving said beam and the sample andsimultaneously detecting, in synchronization with said relative motion,variations in said derived current for indicating changes in the numberof said emitted secondary electrons to detect the presence of the secondmaterial phase in the sample.
 1. A method of detecting, in a sample of afirst phase of a material, the presence of a second, different phase ofthe material, said first and second phases having the same atomic numberZ, which comprises the steps of: impinging said sample with a low energyelectron beam, the electrons in said beam having energies falling withinthe approximate range 0.2-2.0 keV for effecting the emission from saidsample of secondary electrons, substantially all of which have energieswithin the range 0-15 eV, and back-scattered electrons in approximatelyequal numbers from both phases of said material; capturing saidsecondary electrons to the substantial exclusion of said back-scatteredelectrons to thereby derive a current proportional to the number of saidemitted secondary electrons; and relatively moving said beam and thesample and simultaneously detecting, in synchronization with saidrelative motion, variations in said derived current for indicatingchanges in the number of said emitted secondary electrons to detect thepresence of the second material phase in the sample.
 2. The method ofclaim 1 wherein said material is tantalum and said first and secondphases are respectively the Alpha and Beta phases thereof.
 3. The methodof claim 1 wherein said material is tantalum and said first and secondphases are respectively the Beta and Alpha phases thereof.